1. Field of the Invention
The present invention relates to medical devices for monitoring vital signs, e.g., arterial blood pressure.
2. Description of the Related Art
False alarms generated by conventional vital sign monitors can represent up to 90% of all alarms in critical and peri-operative care, and are therefore a source of concern. A variety of factors cause false alarms, one of which is motion-related artifacts. Ultimately false alarms can have a severe impact on the safety of hospitalized patients: they can desensitize medical professionals toward ‘true positive’ alarms, lead them to set dangerously wide alarm thresholds, or even drive them to completely disable alarms. This can have a particularly profound impact in lower-acuity areas of the hospital, i.e. areas outside the intensive care unit (ICU), emergency department (ED), or operating room (OR), where the ratio of medical professionals to patients can be relatively low. In these areas a single medical professional (e.g. a nurse) often has to care for a large number of patients, and necessarily relies on automated alarms operating on vital sign monitors to effectively monitor their patients.
Studies in critical care environments indicate that the majority of false positive alarms are simple ‘threshold alarms’, meaning they are generated when a patient's vital sign exceeds a predetermined threshold. Patient motion can result in a vital sign having an erroneous high or low value, which in turn can trigger the false alarm. In most cases, these alarms lack any real clinical meaning, and go away after about 20 seconds when they are not acknowledged. Alarms can also be artificially induced when a patient is moved or manipulated, or if there is an actual problem with the vital sign monitor. False alarms due to motion-related artifacts are particularly very high when measured from ambulatory patients.
Blood pressure is a vital sign that is particularly susceptible to false alarms. In critical care environments like the ICU and OR, blood pressure can be continuously monitored with an arterial catheter inserted in the patient's radial or femoral artery. Alternatively, blood pressure can be measured intermittently using a pressured cuff and a technique called oscillometry. A vital sign monitor performs both the catheter and cuff-based measurements of blood pressure. Alternatively, blood pressure can be monitored continuously with a technique called pulse transit time (PTT), defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system. PTT has been shown in a number of studies to correlate to systolic (SYS), diastolic (DIA), and mean (MAP) blood pressures. In these studies, PTT is typically measured with a conventional vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and pulse oximetry (SpO2). During a PTT measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent ECG component characterized by a sharp spike called the ‘QRS complex’. The QRS complex indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat and a pressure pulse that follows. SpO2 is typically measured with a bandage or clothespin-shaped sensor that attaches to a patient's finger, and includes optical systems operating in both the red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems that transmits through the patient's finger. Other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger. During a measurement, a microprocessor analyses both red and infrared radiation detected by the photodetector to determine the patient's blood oxygen saturation level and a time-dependent waveform called a photoplethysmograph (‘PPG’). Time-dependent features of the PPG indicate both pulse rate and a volumetric absorbance change in an underlying artery caused by the propagating pressure pulse.
Typical PTT measurements determine the time separating a maximum point on the QRS complex (indicating the peak of ventricular depolarization) and a foot of the optical waveform (indicating the beginning the pressure pulse). PTT depends primarily on arterial compliance, the propagation distance of the pressure pulse (which is closely approximated by the patient's arm length), and blood pressure. To account for patient-dependent properties, such as arterial compliance, PTT-based measurements of blood pressure are typically ‘calibrated’ using a conventional blood pressure cuff and oscillometry. Typically during the calibration process the blood pressure cuff is applied to the patient, used to make one or more blood pressure measurements, and then left on the patient. Going forward, the calibration blood pressure measurements are used, along with a change in PTT, to continuously measure the patient's blood pressure (defined herein as 'cNIBP). PTT typically relates inversely to blood pressure, i.e., a decrease in PTT indicates an increase in blood pressure.
A number of issued U.S. patents describe the relationship between PTT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure an ECG and PPG, which are then processed to determine PTT. U.S. Pat. No. 5,964,701 describes a finger-ring sensor that includes an optical system for detecting a PPG, and an accelerometer for detecting motion.